The inability to find compatible blood for transfusion is not an uncommon occurrence. A procedure has been developed whereby a small amount of 51Cr labeled donor red cells which are incompatible by in-vitro techniques are transfused into the potential recipient and the in-vivo survival of the transfused cells determined. Survival characteristics of these cells allow decisions to be made with greater certainty regarding the safety of standard transfusions. In the patients studied thus far, two cases of the rare "saline auto-agglutinating phenomenon" have been shown to have no significance clinically when appropriate precautions are taken. In addition the suspected incompatibility of group O red cells with a rare "Bombay O" recipient has been documented. In other instances "incompatible" blood for important surgery has been able to be released, whereas previously it would have been difficult to predict the relative hazard of transfusing such blood. Studies of rare or poorly defined red cell antibodies are progressing. It is anticipated that further patients will continue to be included in this study when in-vitro crossmatch incompatibility is encountered.